The present invention relates to a method for forming exposed pad features during lead frame downset for preventing bleed of encapsulant over an integrated circuit exposed pad or slug for utilization as a heat sink.
In the fabrication of integrated circuits, it is often necessary to provide an exposed thermal pad or slug of highly thermally conductive material, such as, for example, copper, which is an integral part of or attached to a lead frame in order to conduct heat away from the semiconductor chip attached to the lead frame. The preferred finish for an exposed pad lead frame is palladium so as to avoid any wet process steps. The purpose of the palladium coating, when used, is set forth in Ser. No. 08/190,729, the contents of which are incorporated herein by reference. The exposed thermal pad is generally defined as the surface of the lead frame opposed to and adjacent to the semiconductor chip. It should be understood that, as an alternative, the lead frame can be made thicker in the region of the thermal pad or with an attached thermal slug. A portion of the processing operation involves encapsulation of the chip and lead frame, this operation requiring that the thermal pad be exposed to the external environment in order to allow for maximum dissipation of heat from the semiconductor chip to the external environment through the thermal pad. During encapsulation, the pad is disposed against the mold surface, preferably under compression against the mold surface, to minimize the likelihood of encapsulant traveling between the mold surface and the thermal pad and thereby over the thermal pad surface.
A problem that arises during the encapsulation as described above is that the compression of the thermal slug against the mold surface is often insufficient to prevent mold bleed wherein the molding material not only encapsulates the lead frame and semiconductor chip, but the molding material also passes over and forms a thin, generally transparent coating over the thermal pad. This coating of encapsulant over the thermal pad reduces the thermal performance of the thermal pad by reducing the exposed pad area. The encapsulant coating over the thermal pad also prevents or reduces the ability to bond or solder to the external thermal sink. In addition, irregular mold bleed makes it difficult to specify manufacturing limits and causes a problem during quality control to judge acceptable and unacceptable criteria. Uncontrolled mold bleed is also a cosmetic defect that may result in unacceptability of an otherwise good product.
In the prior art, this mold bleed has generally been removed by abrading or by chemical action of some type. The abrading or chemical action step involves an economic cost in that the abrading and/or chemical action step must be added. In addition, the abrading and/or chemical action step may damage, contaminate or cause removal of all or part of the palladium coating over the lead frame, thereby minimizing or eliminating the beneficial effects derived from the palladium coating.
Stamp forming tools are used to bend sheet metal to desired shapes. By applying additional force at the bends, it is possible to set the shape. This action is called coining. Coining occurs when pressure is applied to localized areas so as to result in flow of the material. More particularly, this process is used in this application to form lead frame exposed pad features and insure that the pad is flat to the edges in order that the bleed barrier groove is effective. FIG. 19 illustrates stamp forming tool applied to the creation of deep downset exposed pad for a lead frame. The lead frame is placed between a tool 1906 and an anvil 1908. The tool 1906 is moved to bend the lead frame 1910 to the desired shape and coins the lead frame against the anvil 1908. The area of the lead frame 1910 to be coined is controlled by the clearances between the tool 1906 and anvil 1908. Where less coining is desired, greater clearances are allowed and conversely, where more coining is desired, the clearances are reduced. The lead frame exposed pad 1910 includes a wing 1912; typically, there will be 2 to 4 wings. The wing 1912 is formed during the downset of the of the exposed pad. However, typically coining in the area indicated by arrow 1902 and 1904 reaches its limit before the bottom of the exposed pad bottoms out on the anvil. This leads to a problem in that the exposed pad edge 1914 is not flat. Flatness of the exposed pad 1910a is generally required all the way to the edge 1914 in order to slow the mold bleed sufficiently so the groove 1916 can freeze off the bleed.
As illustrated in FIG. 19, the anvil 1908 is a single piece. Since it is difficult to machine the comer 1918 of this single piece without rounding the corner 1918, it is difficult to achieve a nonrounded or sharp angular corner 1920 of lead frame 1910 through coining of the lead frame.
In accordance with the present invention, the above described problems of the prior art are minimized if not eliminated.
Briefly, the above is accomplished by providing a barrier to the encapsulant travel over the thermal pad during device encapsulation. This is accomplished in several ways in accordance with the present invention. In accordance with one group of embodiments of the invention, a groove or grooves of various possible shapes are formed in the lead frame itself by stamping, etching or the like. The groove surrounds all or part of the thermal pad so that any encapsulant traveling toward the center of the thermal pad freezes at the entrance to the groove and therefore acts as a block to the travel of encapsulant over the thermal pad. In some cases, the encapsulant may fill the groove and travels no further. As a second group of embodiments, the groove or grooves can be formed in the mold. As a third group of embodiments, ridges can be placed in the mold to prevent the movement of the encapsulant over the exposed surface of the thermal pad. As a fourth group of embodiments, ridges can be formed on the lead frame surface. In the case of the grooves, the encapsulant is either trapped in the grooves or freezes when entering a groove to inhibit further travel of encapsulant over the thermal pad surface. In the case of the ridges, the ridges rest against the surface of the mold, generally but not necessarily under compression, to act as a barrier to the travel of the encapsulant beyond the ridges and over the surface of the thermal pad. The action is the same as the groove but reversed in implementation.
In those instances wherein the features of the grooves are stamped into the lead frame, there can be greater swelling due to displaced material where grooves meet at a corner. This swelling at the corners results in a failure to maintain the lead frame flat with the mold and permits encapsulant to travel to the thermal pad via longer straight line regions. This problem can be minimized for stamped grooves by avoiding the sharp corners and, instead, providing relatively large corner radii. However, large radii are not often practical. The solution for stamped grooves was to pattern ends of grooves short of the corner or groove line segment such that swelling by displaced groove material created an effective swelling equivalent to the swelling of the straight line segments (see FIGS. 13, 13a, 13b and 14). Etched grooves do not introduce any swelling and can easily be patterned for any shape.
The grooves can take various shapes, examples of these shapes being xe2x80x9cV,xe2x80x9d check, sawtooth, square and rectangular, as well as a concave cup in the case of etching. The grooves will generally have a depth equal to about half of their width though there is no requirement to utilize this dimension ratio.
As a result of the subject invention, bleed control is provided by design into the lead frame or mold to provide lower cost and higher assembly throughout than is obtained by the prior art chemical deflash, mechanical deflash, liquid or tape masking or the pad/slug and by use of tape, chemical or special materials in the package mold.
The present invention requires no chemicals, wet processing or added plating cost. In addition, no processing is required which will degrade package reliability, and no additional assembly processes or materials are required. The palladium solderable finish, when used, is preserved over the lead frame, and the groove provides better ability for quality control inspection for mold bleed.
Additionally, the present invention includes an apparatus for providing a downset for a semiconductor chip. The downset is formed using modular tools instead of a single machine tool. This allows for sharp corners with zero radius bends.